Two-terminal reversibly switchable memory device

ABSTRACT

A memory using mixed valence conductive oxides is disclosed. The memory includes a mixed valence conductive oxide that is less conductive in its oxygen deficient state and a mixed electronic ionic conductor that is an electrolyte to oxygen and promotes an electric field effective to cause oxygen ionic motion.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a continuation of U.S. patent application Ser. No.14/463,518, filed Aug. 19, 2014, which is a continuation of U.S. patentapplication Ser. No. 12/456,627, filed Jun. 18, 2009, now abandoned,which is a continuation of U.S. patent application Ser. No. 11/095,026,filed Mar. 30, 2005, now abandoned, which is a continuation-in-part ofU.S. patent application Ser. No. 10/934,951, filed Sep. 3, 2004, whichissued on May 26, 2009 as U.S. Pat. No. 7,538,338, and which is acontinuation-in-part of U.S. patent application Ser. No. 10/773,549,filed Feb. 6, 2004, which issued on Jul. 25, 2006 as U.S. Pat. No.7,082,052.

BACKGROUND OF THE INVENTION

Field of the Invention

The present invention relates to computer memory and more specificallyto non-volatile memory.

Description of the Related Art

Memory can either be classified as volatile or nonvolatile. Volatilememory is memory that loses its contents when the power is turned off.In contrast, non-volatile memory does not require a continuous powersupply to retain information. Most non-volatile memories use solid-statememory devices as memory elements.

Since the 1960s, a large body of literature has evolved that describesswitching and memory effects in metal-insulator-metal structures withthin insulators. One of the seminal works was “New Conduction andReversible Memory Phenomena in Thin Insulating Films” by J. G. Simmonsand R. R. Verderber in 301 Proc. Roy. Soc. 77-102 (1967), incorporatedherein by reference for all purposes. Although the mechanisms describedby Simmons and Verderber have since been cast into doubt, theircontribution to the field is great.

However, nobody has successfully implemented a metal-insulator-metalstructure into a commercial solid-state memory device. In the text“Oxides and Oxide Films,” volume 6, edited by A. K. Vijh (Marcel Drekker1981) 251-325, incorporated herein by reference for all purposes,chapter 4, written by David P. Oxley, is entirely devoted to “MemoryEffects in Oxide Films.” In that text, Oxley says “It is perhapssaddening to have to record that, even after 10 years of effort, thenumber of applications for these oxide switches is so limited.” He goeson to describe a “need for caution before any application is envisaged.This caution can only be exercised when the physics of the switchingaction is understood; this, in turn, must await a full knowledge of thetransport mechanisms operating in any switch for which a commercial useis envisaged.”

In 2002, over twenty years after writing that chapter, Oxley revisitedthe subject in “The Electroformed metal-insulator-metal structure: Acomprehensive model” by R. E. Thurstans and D. P. Oxley 35 J. Phys. D.Appl. Phys. 802-809, incorporated herein by reference for all purposes.In that article, the authors describe a model that identifies theconduction process as “trap-controlled and thermally activated tunnelingbetween metal islands produced in the forming process.” “Forming” (or“electroforming”) is described as “the localized filamentary movement ofmetallic anode material through the dielectric, induced by the electricfield. Here it is important to note that the evaporated dielectric maycontain voids and departures from stoichiometry. When resultingfilaments through the dielectric carry sufficient current, they ruptureto leave a metal island structure embedded in the dielectric. Electronicconduction is possible through this structure by activating tunneling.”

However, the authors caution, “The forming process is complex andinherently variable. Also tunneling barriers are susceptible to changesin their characteristics when exposed to water vapour, organic speciesand oxygen . . . . Thus, device characteristics can never be expected tobe produced consistently or be stable over long periods withoutpassivation, effective encapsulation and a better understanding of thedynamics of the forming process.”

In seemingly unrelated research, certain conductive metal oxides (CMOs),have been identified as exhibiting a memory effect after being exposedto an electronic pulse. U.S. Pat. No. 6,204,139, issued Mar. 20, 2001 toLiu et al., incorporated herein by reference for all purposes, describessome perovskite materials that exhibit memory characteristics. Theperovskite materials are also described by the same researchers in“Electric-pulse-induced reversible resistance change effect inmagnetoresistive films,” Applied Physics Letters, Vol. 76, No. 19, 8 May2000, and “A New Concept for Non-Volatile Memory: The Electric-PulseInduced Resistive Change Effect in Colossal Magnetoresistive ThinFilms,” in materials for the 2001 Non-Volatile Memory TechnologySymposium, all of which are hereby incorporated by reference for allpurposes.

In U.S. Pat. No. 6,531,371 entitled “Electrically programmableresistance cross point memory” by Hsu et al, incorporated herein byreference for all purposes, resistive cross point memory devices aredisclosed along with methods of manufacture and use. The memory devicecomprises an active layer of perovskite material interposed betweenupper electrodes and lower electrodes.

Similarly, the IBM Zurich Research Center has also published threetechnical papers that discuss the use of metal oxide material for memoryapplications: “Reproducible switching effect in thin oxide films formemory applications,” Applied Physics Letters, Vol. 77, No. 1, 3 Jul.2000, “Current-driven insulator-conductor transition and nonvolatilememory in chromium-doped SrTiO₃ single crystals,” Applied PhysicsLetters, Vol. 78, No. 23, 4 Jun. 2001, and “Electric currentdistribution across a metal-insulator-metal structure during bistableswitching,” Journal of Applied Physics, Vol. 90, No. 6, 15 Sep. 2001,all of which are hereby incorporated by reference for all purposes.

There are continuing efforts to incorporate solid state memory devicesinto a commercial non-volatile RAM.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention may best be understood by reference to the followingdescription taken in conjunction with the accompanying drawings, inwhich:

FIG. 1A depicts a perspective view of an exemplary cross point memoryarray employing a single layer of memory;

FIG. 1B depicts a perspective view of an exemplary stacked cross pointmemory array employing four layer of memory;

FIG. 2A depicts a plan view of selection of a memory cell in the crosspoint array depicted in FIG. 1A;

FIG. 2B depicts a perspective view of the boundaries of the selectedmemory cell depicted in FIG. 2A;

FIG. 3 depicts a generalized cross-sectional representation of a memorycell that can be used in a transistor memory array;

FIG. 4A depicts a block diagram of a representative implementation of anexemplary 1 MB memory;

FIG. 4B depicts a block diagram of an exemplary memory that includessensing circuits that are capable of reading multiple bits;

FIG. 5A depicts a block diagram representing the basic components of oneembodiment of a memory element;

FIG. 5B depicts a block diagram of the memory element of FIG. 5A in atwo-terminal memory cell;

FIG. 5C depicts a block diagram of the memory element of FIG. 5A in athree-terminal memory cell;

FIG. 6A depicts a block diagram of the memory cell of FIG. 5B whereoxygen movement results in a low conductivity oxide;

FIG. 6B depicts a block diagram of the memory cell of FIG. 5B where alow conductivity oxide is self-limiting;

FIG. 7 depicts a block diagram of a two-terminal memory cell usinganother memory element embodiment;

FIG. 8A depicts a block diagram of the memory cell of FIG. 7 where a lowconductivity region is created in a mixed valence oxide; and

FIG. 8B depicts a block diagram of the memory cell of FIG. 8A thatincludes an oxygen repository.

It is to be understood that, in the drawings, like reference numeralsdesignate like structural elements. Also, it is understood that thedepictions in the FIGS. are not necessarily to scale.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

In the following description, numerous specific details are set forth toprovide a thorough understanding of the present invention. It will beapparent, however, to one skilled in the art that the present inventionmay be practiced without some or all of these specific details. In otherinstances, well known process steps have not been described in detail inorder to avoid unnecessarily obscuring the present invention.

The Memory Array

Conventional nonvolatile memory requires three terminal MOSFET-baseddevices. The layout of such devices is not ideal, usually requiring anarea of at least 8f² for each memory cell, where f is the minimumfeature size. However, not all memory elements require three terminals.If, for example, a memory element is capable of changing its electricalproperties (e.g., resistivity) in response to a voltage pulse, only twoterminals are required. With only two terminals, a cross point arraylayout that allows a single cell to be fabricated to a size of 4f² canbe utilized.

FIG. 1A depicts a perspective view of an exemplary cross point memoryarray 100 employing a single layer of memory. A bottom layer ofx-direction conductive array lines 105 is orthogonal to a top layer ofy-direction conductive array lines 110. The x-direction conductive arraylines 105 act as a first terminal and the y-direction conductive arraylines 110 act as a second terminal to a plurality of memory plugs 115,which are located at the intersections of the conductive array lines 105and 110. The conductive array lines 105 and 110 are used to both delivera voltage pulse to the memory plugs 115 and carry current through thememory plugs 115 in order to determine their resistive states.

Conductive array line layers 105 and 110 can generally be constructed ofany conductive material, such as aluminum, copper, tungsten or certainceramics. Depending upon the material, a conductive array line wouldtypically cross between 64 and 8192 perpendicular conductive arraylines. Fabrication techniques, feature size and resistivity of materialmay allow for shorter or longer lines. Although the x-direction andy-direction conductive array lines can be of equal lengths (forming asquare cross point array) they can also be of unequal lengths (forming arectangular cross point array), which may be useful if they are madefrom different materials with different resistivities.

FIG. 2A illustrates selection of a memory cell 205 in the cross pointarray 100. The point of intersection between a single x-directionconductive array line 210 and a single y-direction conductive array line215 uniquely identifies the single memory cell 205. FIG. 2B illustratesthe boundaries of the selected memory cell 205. The memory cell is arepeatable unit that can be theoretically extended in one, two or eventhree dimensions. One method of repeating the memory cells in thez-direction (orthogonal to the x-y plane) is to use both the bottom andtop surfaces of conductive array lines 105 and 110, creating a stackedcross point array.

FIG. 1B depicts an exemplary stacked cross point array 150 employingfour memory layers 155, 160, 165, and 170. The memory layers aresandwiched between alternating layers of x-direction conductive arraylines 175, 180 and 185 and y-direction conductive array lines 190 and195 such that each memory layer 155, 160, 165, and 170 is associatedwith only one x-direction conductive array line layer and oney-direction conductive array line layer. Although the top conductivearray line layer 185 and bottom conductive array line layer 175 are onlyused to supply voltage to a single memory layer 155 and 170, the otherconductive array line layers 180, 190, and 195 can be used to supplyvoltage to both a top and a bottom memory layer 155, 160, 165, or 170.

Referring back to FIG. 2B, the repeatable cell that makes up the crosspoint array 100 can be considered to be a memory plug 255, plus ½ of thespace around the memory plug, plus ½ of an x-direction conductive arrayline 210 and ½ of a y-direction conductive array line 215. Of course, ½of a conductive array line is merely a theoretical construct, since aconductive array line would generally be fabricated to the same width,regardless of whether one or both surfaces of the conductive array linewas used. Accordingly, the very top and very bottom layers of conductivearray lines (which use only one surface) would typically be fabricatedto the same size as all other layers of conductive array lines.

One benefit of the cross point array is that the active circuitry thatdrives the cross point array 100 or 150 can be placed beneath the crosspoint array, therefore reducing the footprint required on asemiconductor substrate. However, the cross point array is not the onlytype of memory array that can be used with a two-terminal memoryelement. For example, a two-dimensional transistor memory array canincorporate a two-terminal memory element. While the memory element insuch an array would be a two-terminal device, the entire memory cellwould be a three-terminal device.

FIG. 3 is a generalized diagrammatic representation of a memory cell 300that can be used in a transistor memory array. Each memory cell 300includes a transistor 305 and a memory plug 310. The transistor 305 isused to permit current from the data line 315 to access the memory plug310 when an appropriate voltage is applied to the select line 320, whichis also the transistor's gate. The reference line 325 might span twocells if the adjacent cells are laid out as the mirror images of eachother.

Memory Chip Configuration

FIG. 4A is a block diagram of a representative implementation of anexemplary 1 MB memory 400A. Physical layouts might differ, but eachmemory bit block 405 can be formed on a separate portion of asemiconductor substrate. Input signals into the memory 400A can includean address bus 430, a control bus 440, some power supplies 450(typically Vcc and ground—the other signals of bus 450 can be internallygenerated by the 1 MB memory 400A), and a data bus 460. The control bus440 typically includes signals to select the chip, to signal whether aread or write operation should be performed, and to enable the outputbuffers when the chip is in read mode. The address bus 430 specifieswhich location in the memory array is accessed—some addresses going tothe X block 470 (typically including a predecoder and an X-decoder) toselect one line out of the horizontal array lines. The other addressesgo to a Y block 480 (typically including a predecoder and a Y-decoder)to apply the appropriate voltage on specific vertical lines. Each memorybit block 405 operates on one line of the memory chip data bus 460.

The reading of data from a memory array 420 is relativelystraightforward: an x-line is energized, and current is sensed by thesensing circuits 410 on the energized y-lines and converted to bits ofinformation.

FIG. 4B is a block diagram of an exemplary memory 400B that includessensing circuits 415 that are capable of reading multiple bits. Thesimultaneous reading of multiple bits involves sensing current frommultiple y-lines simultaneously.

During a write operation, the data is applied from the data bus 460 tothe input buffers and data drivers 490 to the selected vertical lines,or bit lines. Specifically, when binary information is sent to thememory chip 400B, it is typically stored in latch circuits within thecircuits 495. Within the circuits 495, each y-line can either have anassociated driver circuit or a group of y-lines can share a singledriver circuit if the non-selected lines in the group do not cause theunselected memory plugs to experience any change in resistance,typically by holding the non-selected lines to a constant voltage. As anexample, there may be 1024 y-lines in a cross point array, and the pageregister may include 8 latches, in which case the y-block would decode 1out of 128 y-lines and connect the selected lines to block 495. Thedriver circuit then writes the 1 or 0 to the appropriate memory plug.The writing can be performed in multiple cycles. In a scheme describedin PCT Patent Application No. PCT/US04/13836, filed May 3, 2004,incorporated herein by reference, all the is can be written during afirst cycle and all the 0s can be written during a second cycle. Asdescribed below, certain memory plugs can have multiple stable distinctresistive states. With such multi-level resistance memory plugs, drivercircuits could program, for example, states of 00, 01, 10 or 11 byvarying write voltage magnitude or pulse length.

It is to be noted that such an architecture can be expanded to create amemory where one array handles all the bits of the data bus, as opposedto having multiple arrays, or memory bit blocks as described above. Forexample, if the data bus, or memory data organization, also called datawidth, is 16-bit wide, the y-block of one cross point array can be madeto decode 16 lines simultaneously. By applying the techniques ofsimultaneous reads and 2-cycle writes, such a memory chip with only onearray can read and program 16-bit words.

Memory Plug

Each memory plug contains layers of materials that may be desirable forfabrication or functionality. For example, a non-ohmic characteristicthat exhibit a very high resistance regime for a certain range ofvoltages (V_(NO−) to V_(NO+)) and a very low resistance regime forvoltages above and below that range might be desirable. In a cross pointarray, a non-ohmic characteristic could prevent leakage during reads andwrites if half of both voltages were within the range of voltagesV_(NO−) to V_(NO+). If each conductive array line carried ½ V_(W), thecurrent path would be the memory plug at the intersection of the twoconductive array lines that each carried ½ V_(W). The other memory plugswould exhibit such high resistances from the non-ohmic characteristicthat current would not flow through the half-selected plugs.

A non-ohmic device might be used to cause the memory plug to exhibit anon-linear resistive characteristic. Exemplary non-ohmic devices includethree-film metal-insulator-metal (MIM) structures and back-to-backdiodes in series. Separate non-ohmic devices, however, may not benecessary. Certain fabrications of the memory plug can cause a non-ohmiccharacteristic to be imparted to the memory cell. While a non-ohmiccharacteristic might be desirable in certain arrays, it may not berequired in other arrays.

Electrodes will typically be desirable components of the memory plugs, apair of electrodes sandwiching the memory element. If the only purposeof the electrodes is as a barrier to prevent metal inter-diffusion, thena thin layer of non-reactive metal, e.g. TiN, TaN, Pt, Au, and certainmetal oxides could be used. However, electrodes may provide advantagesbeyond simply acting as a metal inter-diffusion barrier. Electrodes(formed either with a single layer or multiple layers) can performvarious functions, including to: prevent the diffusion of metals,oxygen, hydrogen and water; act as a seed layer in order to form a goodlattice match with other layers; act as adhesion layers; reduce stresscaused by uneven coefficients of thermal expansion; and provide otherbenefits. Additionally, the choice of electrode layers can affect thememory effect properties of the memory plug and become part of thememory element.

The “memory element electrodes” are the electrodes (or, in certaincircumstances, the portion of the conductive array lines) that thememory elements are sandwiched in-between. As used herein, memoryelement electrodes are what allow other components to be electricallyconnected to the memory element. It should be noted that both crosspoint arrays and transistor memory arrays have exactly two memoryelement electrodes since the memory plug has exactly two terminals,regardless of how many terminals the memory cell has. Those skilled inthe art will appreciate that a floating gate transistor, if used as amemory element, would have exactly three memory element electrodes(source, drain and gate).

Memory Effect

The memory effect is a hysteresis that exhibits a resistive state changeupon application of a voltage while allowing non-destructive reads. Anon-destructive read means that the read operation has no effect on theresistive state of the memory element. Measuring the resistance of amemory cell is generally accomplished by detecting either current afterthe memory cell is held to a known voltage, or voltage after a knowncurrent flows through the memory cell. Therefore, a memory cell that isplaced in a high resistive state R₀ upon application of −V_(W) and a lowresistive state R₁ upon application of +V_(W) should be unaffected by aread operation performed at −V_(R) or +V_(R). In such materials a writeoperation is not necessary after a read operation. It should beappreciated that the magnitude of |−V_(R)| does not necessarily equalthe magnitude of |+V_(R)|.

Furthermore, it is possible to have a memory cell that can be switchedbetween resistive states with voltages of the same polarity. Forexample, in the paper “The Electroformed metal-insulator-metalstructure: a comprehensive model,” already incorporated by reference,Thurstans and Oxley describe a memory that maintains a low resistivestate until a certain V_(P) is reached. After V_(P) is reached theresistive state can be increased with voltages. After programming, thehigh resistive state is then maintained until a V_(T) is reached. TheV_(T) is sensitive to speed at which the program voltage is removed fromthe memory cell. In such a system, programming R₁ would be accomplishedwith a voltage pulse of V_(P), programming R₀ would be accomplished witha voltage pulse greater than V_(P), and reads would occur with avoltages below V_(T). Intermediate resistive states (for multi-levelmemory cells) are also possible.

The R₁ state of the memory plug may have a best value of 10 kΩ to 100kΩ. If the R₁ state resistance is much less than 10 kΩ, the currentconsumption will be increased because the cell current is high, and theparasitic resistances will have a larger effect. If the R₁ state valueis much above 100 kΩ, the RC delays will increase access time. However,workable single state resistive values may also be achieved withresistances from 5 kΩ to 1MΩ and beyond with appropriate architecturalimprovements. Typically, a single state memory would have theoperational resistances of R₀ and R₁ separated by a factor of 10.

Since memory plugs can be placed into several different resistivestates, multi-bit resistive memory cells are possible. Changes in theresistive property of the memory plugs that are greater than a factor of10 might be desirable in multi-bit resistive memory cells. For example,the memory plug might have a high resistive state of R₀₀, a medium-highresistive state of R₀₁, a medium-low resistive state of R₁₀ and a lowresistive state of R₁₁. Since multi-bit memories typically have accesstimes longer than single-bit memories, using a factor greater than a 10times change in resistance from R₁₁ to R₀₀ is one way to make amulti-bit memory as fast as a single-bit memory. For example, a memorycell that is capable of storing two bits might have the low resistivestate be separated from the high resistive state by a factor of 100. Amemory cell that is capable of storing three or four bits of informationmight require the low resistive state be separated from the highresistive state by a factor of 1000.

Creating the Memory Effect with Tunneling

Tunneling is a process whereby electrons pass through a barrier in thepresence of an electric field. Tunneling is exponentially dependant on abarrier's width and the square root of its height. Barrier height istypically defined as the potential difference between the Fermi energyof a first conducting material and the band edge of a second insulatingmaterial. The Fermi energy is that energy at which the probability ofoccupation of an electron state is 50%. Barrier width is the physicalthickness of the insulating material.

The barrier height might be modified if carriers or ions are introducedinto the second material, creating an additional electric field. Abarrier's width can be changed if the barrier physically changes shape,either growing or shrinking. In the presence of a high electric field,both mechanisms could result in a change in conductivity.

Although the following discussion focuses mainly on purposefullymodifying the barrier width, those skilled in the art will appreciatethat other mechanisms can be present, including but not limited to:barrier height modification, carrier charge trapping space-chargelimited currents, thermionic emission limited conduction, and/orelectrothermal Poole-Frenkel emission.

FIG. 5A is a block diagram representing the basic components of oneembodiment of a memory element 500, FIG. 5B is a block diagram of thememory element 500 in a two-terminal memory cell, and FIG. 5C is a blockdiagram of the memory element embodiment of FIG. 5A in a three-terminalmemory cell.

FIG. 5A shows an electrolytic tunnel barrier 505 and an ion reservoir510, two basic components of the memory element 500. FIG. 5B shows thememory element 500 between a top memory electrode 515 and a bottommemory electrode 520. The orientation of the memory element (i.e.,whether the electrolytic tunnel barrier 505 is near the top memoryelectrode 515 or the bottom memory electrode 520) may be important forprocessing considerations, including the necessity of seed layers andhow the tunnel barrier reacts with the ion reservoir 510 duringdeposition. FIG. 5C shows the memory element 500 oriented with theelectrolytic tunnel barrier 505 on the bottom in a three-terminaltransistor device, having a source memory element electrode 525, gatememory element electrode 530 and a drain memory element electrode 535.In such an orientation, the electrolytic tunnel barrier 505 could alsofunction as a gate oxide.

Referring back to FIG. 5A, the electrolytic tunnel barrier 505 willtypically be between 10 and less than 50 angstroms. If the electrolytictunnel barrier 505 is much greater than 50 angstroms, then the voltagethat is required to create the electric field necessary to moveelectrons through the memory element 500 via tunneling becomes too highfor most electronic devices. Depending on the electrolytic tunnelbarrier 505 material, a preferred electrolytic tunnel barrier 505 widthmight be between 15 and 40 angstroms for circuits where rapid accesstimes (on the order of tens of nanoseconds, typically below 100 ns) insmall dimension devices (on the order of hundreds of nanometers) aredesired.

Fundamentally, the electrolytic tunnel barrier 505 is an electronicinsulator and an ionic electrolyte. As used herein, an electrolyte isany medium that provides an ion transport mechanism between positive andnegative electrodes. Materials suitable for some embodiments includevarious metal oxides such as Al₂O₃, Ta₂O₅, HfO₂ and ZrO₂. Some oxides,such as zirconia might be partially or fully stabilized with otheroxides, such as CaO, MgO, or Y₂O₃, or doped with materials such asscandium.

The electrolytic tunnel barrier 505 will typically be of very highquality, being as uniform as possible to allow for predictability in thevoltage required to obtain a current through the memory element 500.Although atomic layer deposition and plasma oxidation are examples ofmethods that can be used to create very high quality tunnel barriers,the parameters of a particular system will dictate its fabricationoptions. Although tunnel barriers can be obtained by allowing a reactivemetal to simply come in contact with an ion reservoir 510, as describedin PCT Patent Application No. PCT/US04/13836, filed May 3, 2004, alreadyincorporated herein by reference, such barriers may be lacking inuniformity, which may be important in some embodiments. Accordingly, ina preferred embodiment of the invention the tunnel barrier does notsignificantly react with the ion reservoir 510 during fabrication.

With standard designs, the electric field at the tunnel barrier 505 istypically high enough to promote tunneling at thicknesses between 10 and50 angstroms. The electric field is typically higher than at otherpoints in the memory element 500 because of the relatively high serialelectronic resistance of the electrolytic tunnel barrier 505. The highelectric field of the electrolytic tunnel barrier 505 also penetratesinto the ion reservoir 510 at least one Debye length. The Debye lengthcan be defined as the distance which a local electric field affectsdistribution of free charge carriers. At an appropriate polarity, theelectric field within the ion reservoir 510 causes ions (which can bepositively or negatively charged) to move from the ion reservoir 510through the electrolytic tunnel barrier 505, which is an ionicelectrolyte.

The ion reservoir 510 is a material that is conductive enough to allowcurrent to flow and has mobile ions. The ion reservoir 510 can be, forexample, an oxygen reservoir with mobile oxygen ions. Oxygen ions arenegative in charge, and will flow in the direction opposite of current.

FIG. 6A is a block diagram where a redox reaction between the oxygenreservoir 635 and a complementary reservoir 615 results in a lowconductivity oxide 640 and an oxygen-depleted low conductivity region620. In the case where the ion reservoir 510 is made up of negativeoxygen ions, an appropriate complementary reservoir 615 would bepositively charged ions. Additionally, the complementary reservoir 615for the embodiment depicted in FIG. 6A should be conductive in itsnon-oxidized state and exhibit low conductivity in its oxidized state.Accordingly, many conductive metals (including alkali metals, alkalineearth metals, transition metals and other metals) could act as acomplementary reservoir 615. For ease of fabrication, the complimentaryreservoir 615 may be the non-oxidized form of the same material that isused for the electrolytic tunnel barrier 505.

When an electric field is applied across the electrolytic tunnel barrier505, the electric field would penetrate at least one Debye length intothe oxygen reservoir 635. The negatively charged oxygen ions migratethrough the electrolytic tunnel barrier 505 to combine with positivelycharged metal ions in the complementary reservoir 615, creating a lowconductivity oxide 640. This low conductivity oxide 640 is cumulativewith the electrolytic tunnel barrier 505, forcing electrons to tunnel agreater distance to reach the conductive complimentary reservoir 615.Because of the exponential effect of bather width on tunneling, the lowconductivity oxide 640 can be just a few angstroms wide and still have avery noticeable effect on the memory element's effective resistance.

Those skilled in the art will appreciate that redox reaction can occurat either the top or bottom surface of the electrolytic tunnel barrier505. The low conductivity oxide 640 will form at the top of theelectrolytic tunnel barrier 505 if the mobility of the complementaryions is greater than the mobility of the oxygen ions through theelectrolytic tunnel barrier 505. Conversely, if the mobility of oxygenions is greater than the mobility of the complementary ions through theelectrolytic tunnel barrier 505, then the low conductivity oxide 640will form at the bottom of the electrolytic tunnel barrier 505.

The stability of metal oxides will depend on its activation energy.Reversing the redox reaction for many metal oxides, such as Hf and Al,requires a great amount of energy, making such high activation energycells convenient for use as one-time programmable memories. Oxides withlow activation energy, such as RuO_(x) and CuO_(x), are usuallydesirable for reprogrammable memories.

One optimization would be to use the polarity that is less sensitive toread disturbs during reads. For write once memory this may becomplementary to the write polarity. Alternatively, alternating readpolarities can be used. Another optimization for certain embodimentscould be to limit the size of the complementary reservoir 615.

FIG. 6B is a block diagram where the complimentary reservoir 615 isfabricated to be self-limiting. Since only a small amount of thecomplementary reservoir 615 is deposited, the amount of positive ionsavailable to combine with the free oxygen ions is limited. Once all thefree ions in the complimentary reservoir 615 are consumed, no more lowconductivity oxide 640 could be formed.

In most cases the effective width of the tunneling barrier is limitedonly by the availability of ions in the reservoirs 615 and 635. Sincemany different barrier widths can be formed multiple bits per cell canbe easily implemented with different resistive states.

Referring back to FIG. 5A, certain ion reservoirs 510 have the physicalproperty of being less conductive in an oxygen-deficient state. Someexamples of materials that have mobile oxygen ions and are lessconductive in an oxygen-deficient state include certain perovskites (aperovskite generally being in the form of an ABX₃ structure, where A hasan atomic size of 1.0-1.4 Å and B has an atomic size of 0.45-0.75 Å forthe case where X is either oxygen or fluorine) such as SrRuO₃ (SRO),Pr_(0.7)Ca_(0.3)MnO₃, Pr_(0.5)Ca_(0.5)MnO₃ and other PCMOs. Many ofthese ion reservoirs 510 are potentially mixed valence oxides. Forexample, PCMO might be more conductive when its manganese ion is in itsMn³⁺ state, but less conductive when its manganese ion is in its Mn⁴⁺state.

Accordingly, as shown in FIG. 6A, certain oxygen reservoirs 635 willadditionally form an oxygen-depleted low conductivity region 620 thatalso adds to the memory effect. Those skilled in the art will appreciatethat either the oxygen-depleted low conductivity region 620 or the lowconductivity oxide 640 may independently be sufficient to create anacceptable memory effect or, if the conduction mechanisms are different(e.g., small polaron hopping through the oxygen-depleted lowconductivity region 620 and tunneling through the low conductivity oxide640) one mechanism may even dominate the overall conduction through thememory element 500. Accordingly, memory cells can be designed to takeadvantage of only one phenomenon or the other or both.

Creating the Memory Effect with Oxygen Depletion

FIG. 7 is a block diagram representing another embodiment of a memoryelement 700 in a two-terminal memory cell where an oxygen-depleted lowconductivity region in an otherwise conductive material creates themajority of the memory effect. FIG. 7 shows a mixed valence oxide 710and a mixed electronic ionic conductor 705, two basic components of thememory element 700 between a top memory electrode 515 and a bottommemory electrode 520. As with the embodiment of FIG. 5A, the orientationof the memory element may be important for processing considerations. Itshould be appreciated that the memory element can also be used in athree-terminal memory cell, similar to what is depicted in FIG. 5C.

In these embodiments, ion deficiency (which, in the embodiment of FIG.7, is oxygen) will cause an otherwise conductive material to become lessconductive. The mixed valence oxide 710 will generally be crystalline,either as a single crystalline structure or a polycrystalline structure.In one specific embodiment the crystalline structure maintains its basiccrystallinity (with some degree of deformation) in both valence states.By maintaining its crystallinity, both the physical stresses on thememory element may be reduced and the reversibility of the process maybe easier to achieve.

The mixed electronic ionic conductor 705 is similar, and in some casesidentical, to the electrolytic tunnel barrier 505 of FIGS. 6A and 6B.Like the electrolytic tunnel barrier 505, the mixed electronic ionicconductor 705 is both an electrolyte and creates a high electric fieldthat promotes ionic movement. However, whether the mixed electronicionic conductor 705 promotes actual tunneling is not critical.

In FIG. 8A the mixed electronic ionic conductor 705 also acts as anoxygen repository, temporarily holding oxygen until an opposite polarityvoltage pulse pushes the oxygen back into the mixed valence oxide 710.In FIG. 8B a separate oxygen repository 715 layer is used to hold theoxygen. The oxygen repository 715 may be identical to the previouslydescribed complementary reservoir 615 or even certain types of oxygenreservoirs 635 such as IrO_(x). If a redox reaction creates an oxide inthe oxygen repository 715, the activation energy required todisassociate the oxygen from the oxide will influence whether the memoryis used as a one time programmable memory or a rewritable memory.

In one specific embodiment that is similar to an inverted embodiment ofwhat is shown in FIG. 8A, the bottom electrode 520 might be a 500Angstrom layer of platinum, DC magnetron sputtered with 180 wattsapplied to a platinum target in 4 mTorr of argon at 450° C. and thencooled in-situ for at least 10 minutes in the sputter ambient gasenvironment of 4 mTorr of argon.

The mixed valence oxide 710 might be a 500 Angstrom layer of a PCMOperovskite, RF magnetron sputtered in 10 mTorr of argon at 550° C. byapplying 120 watts to a Pr_(0.7)Ca_(0.3)MnO₃ target (made with hotisostatic pressing or HIP), afterwards cooled in-situ for 10 minutes inthe sputter ambient gas environment of 10 mTorr of argon, then cooledfor another 10 minutes in a load lock chamber at 600 Torr of oxygen.

The mixed electronic ionic conductor 705 might be 20 or 30 Angstroms ofsome type of AlO_(x), RF magnetron sputtered in 4 mTorr of argon with 1%oxygen at 300° C. by applying 150 watts to an Al₂O₃ target (also madewith HIP), and then annealed for 30 minutes at 250° C. in the sputterambient gas environment of 4 mTorr of argon with 1% O₂.

If an embodiment similar to FIG. 8B were desired, an oxygen repository715 of 200 Angstroms of aluminum metal could be DC magnetron sputteredwith 250 watts applied to an aluminum target in 4 mTorr of argon at 25°C.

The top electrode 515 might be 500 Angstroms of platinum, DC magnetronsputtered with 180 watts applied to a platinum target in 4 mTorr ofargon at 25° C.

Concluding Remarks

Although the invention has been described in its presently contemplatedbest mode, it is clear that it is susceptible to numerous modifications,modes of operation and embodiments, all within the ability and skill ofthose familiar with the art and without exercise of further inventiveactivity. For example, although the ion reservoir was described as beingnegative in connection with the oxygen reservoir, a positively chargedion reservoir may have the same functionality, as long as the otherphysical requirements of the specific embodiments are met. Furthermore,while the theories provided above are one possible explanation of howthe various materials interact, the inventors do not wish to be bound byany theoretical explanation. Accordingly, that which is intended to beprotected by Letters Patent is set forth in the claims and includes allvariations and modifications that fall within the spirit and scope ofthe claims.

What is claimed is:
 1. An apparatus, comprising: a first electrode; asecond electrode; and a memory element (ME) comprising tantalum, thememory element positioned between the first and second electrodes, theME electrically coupled in series with the first and second electrodes,the ME comprising: a first ion reservoir comprising mobile oxygen ions,the first ion reservoir in contact with the first electrode; a tunnelbarrier in contact with the first ion reservoir, the tunnel barrier madefrom a material that is an electrolyte to the mobile oxygen ions and ispermeable by the mobile oxygen ions responsive to a first voltageapplied across the first and second electrodes, and wherein the tunnelbarrier is coupled in series with the first ion reservoir; and a secondreservoir in contact with the tunnel barrier and positioned between thesecond electrode and the tunnel barrier, the second reservoir operativeas a repository for the mobile oxygen ions.
 2. The apparatus of claim 1,wherein the ME has an electrical conductivity that can be reversiblyswitched between a first conductivity and a second conductivity that isgreater than the first conductivity as a function of mobile oxygen ionmigration.
 3. The apparatus of claim 1, wherein the mobile oxygen ionsin the first ion reservoir are operative to change a conductivity of thefirst ion reservoir from a relatively high conductivity to a relativelylow conductivity when the mobile oxygen ions are transported into thetunnel barrier or from a relatively low conductivity to a relativelyhigh conductivity when the mobile oxygen ions are transported back intothe first ion reservoir.
 4. The apparatus of claim 1, wherein the firstvoltage is operative to generate a first electric field in the tunnelbarrier and a second electric field in the first ion reservoir, thefirst electric field having a greater magnitude than the second electricfield and the first electric field extending into the first ionreservoir by a distance of at least one Debye length.
 5. The apparatusof claim 1, further comprising: a line coupled to the first electrode;and a component coupled to the line, the component configured toenergize the line to apply a second voltage across the first and secondelectrodes, the second voltage operative to generate a read currenthaving a magnitude that is indicative of a present conductivity of theME, wherein applying the second voltage is non-destructive to thepresent conductivity of the ME.
 6. The apparatus of claim 5, wherein amagnitude of the second voltage is less than a magnitude of the firstvoltage.
 7. The apparatus of claim 5, wherein a polarity of the secondvoltage is different than a polarity of the first voltage.
 8. Theapparatus of claim 5, wherein the present conductivity is indicative ofstored data.
 9. The apparatus of claim 8, wherein the stored data isretained in an absence of electrical power.
 10. The apparatus of claim5, wherein the first voltage is applied as a voltage pulse.
 11. Theapparatus of claim 5, wherein the second voltage is applied as a voltagepulse.
 12. The apparatus of claim 1, the ME comprises a tantalum oxidecompound.
 13. The apparatus of claim 12, wherein the tantalum oxidecomprises tantalum pentoxide.
 14. The apparatus of claim 1, wherein atleast a portion of the tantalum of the ME is located in the tunnelbarrier.
 15. The apparatus of claim 1, wherein at least a portion of thetantalum of the ME is located in the second reservoir.
 16. An apparatus,comprising: a first electrode; a second electrode; and a memory element(ME) positioned between the first and second electrodes, the MEelectrically coupled in series with the first and second electrodes, theME comprising: a first ion reservoir comprising mobile ions, the firstion reservoir coupled with the first electrode; and a tunnel barriercoupled with the first ion reservoir and the second electrode, whereinthe tunnel barrier to allow the mobile ions of the first ion reservoirto tunnel through the tunnel barrier responsive to a first voltageapplied across the first electrode and the second electrode, and whereinthe tunnel barrier is coupled in series with the first ion reservoir.17. The apparatus of claim 16, wherein the ME comprises a tantalum oxidecompound.
 18. The apparatus of claim 16, wherein the ME comprisestantalum, wherein at least a portion of the tantalum of the ME islocated in the tunnel barrier.
 19. The apparatus of claim 16, furthercomprising a second reservoir in contact with the tunnel barrier andpositioned between the second electrode and the tunnel barrier, thesecond reservoir operative as a repository for the mobile ions, whereinthe ME comprises tantalum, wherein at least a portion of the tantalum ofthe ME is located in the second reservoir.
 20. The apparatus of claim16, further comprising: a line coupled to the first electrode; and acomponent coupled to the line, the component configured to energize theline to apply a second voltage across the first and second electrodes,the second voltage operative to generate a read current having amagnitude that is indicative of a present conductivity of the ME,wherein applying the second voltage is non-destructive to the presentconductivity of the ME; and wherein at least one of the first voltage orthe second voltage is applied as a voltage pulse.